This invention relates to a three-dimensional field for calibration for use in exactly measuring internal parameters, such as a principal point position, a screen distance (focal length) and a distortion parameter of a lens, necessary to correct an image distortion of a camera. This invention also relates to a method of photographing a three-dimensional field for calibration to obtain a calibration image for use in measuring internal parameters of a camera.
Conventionally, in the field of photogrammetry or photographic measurement it has been important to obtain an image with less aberration. For this purpose, in these fields a high-accuracy lens with small aberration has been used for a photographic camera, as disclosed in JP-A-2001-280956. Further, in the field of phtogrammetry, each of the multiple points positioned and precisely measured in three dimension is measured in plural directions and then the internal parameters, such as a principal point position, a screen distance (focal length) and a distortion parameter, of a camera are analytically obtained, while, in the field of photographic measurement, the internal parameters of a fabricated camera are obtained by precisely measuring the camera.
Recently a digital camera with an image pick up device (charge coupled image sensor) is in widespread use. There are provided a number of models of the digital camera having a so called zoom lens capable of varying a focal length thereof. Use of a zoom lens is effective to adjust an area or size that the image of an object to be measured occupies in an image projected on a film because it is unnecessary to move the camera relative to the object to be measured. Accordingly, it will become easy to perform a three-dimensional image measurement if the zoom lens can be applied to the photographic measurement. However, there is a problem in use of the zoom lens that a calibration work becomes complicated due to a variation of internal parameters of the camera depending on a change in the focal length of the zoom lens.
Specifically, when a three-dimensional field for calibration having multiplicity of points (targets) that are precisely measured and positioned thereon in three dimensions is taken with a camera to be calibrated, it is preferable that the three-dimensional field for calibration is taken so as to occupy fully an image projected on a film or CCD. Unfortunately, the size of an object to be photographed varies according to the variation in focal length of the zoom lens. For photographing the three-dimensional field for calibration so as to fully occupy the projected image, it is therefore necessary to shorten the distance between the three-dimensional field for calibration and camera under a wide-angle condition, while it is necessary to lengthen it under a zooming condition. Under some zooming conditions, the required distance between the three-dimensional field for calibration and camera is, for example, about 10 to 20 meters, which requires an expansive space for the three-dimensional field for calibration to be placed. It is therefore a further problem that a space available for photographing the three-dimensional field for calibration and performing an associated calibration work calibrate is limited.
In photographing the three-dimensional field for calibration, it is also preferable that the targets located in the three-dimensional field for calibration are derived in a mode in which those targets can be clearly recognized and taken with a camera to be calibrated. On the other hand, in many cases, near the three-dimensional field for calibration are placed tools or instruments regardless of calibration and they may be projected as inutile objects on an photographed image of the three-dimensional field for calibration. Accordingly, it is also a problem that there is provided a noise or bar for a calibration work performed every focal length of a zoom lens.